Based on the current technology, the principle that we can hear sounds is that the vibration transferred through the air in our external acoustic meatus, reaches to the ear drum, and the vibration in the ear drum drives our auditory nerves, makes us feel the acoustic vibrations. The current bone conduction speakers are transferring vibrations through our skin, subcutaneous tissues and bones to our auditory nerves, making us hear the sounds.
When the current bone conduction speakers are working, with the vibration of the vibration board, the shell body, fixing the vibration board with some fixers, will also vibrate together with it, thus, when the shell body is touching our post auricles, cheeks, forehead or other parts, the vibrations will be transferred through bones, making us hear the sounds clearly.
However, the frequency response curves generated by the bone conduction speakers with current vibration devices are shown as the two solid lines in FIG. 4. In ideal conditions, the frequency response curve of a speaker is expected to be a straight line, and the top plain area of the curve is expected to be wider, thus the quality of the tone will be better, and easier to be perceived by our ears. However, the current bone conduction speakers, with their frequency response curves shown as FIG. 4, have overtopped resonance peaks either in low frequency area or high frequency area, which has limited its tone quality a lot. Thus, it is very hard to improve the tone quality of current bone conduction speakers containing current vibration devices. The current technology needs to be improved and developed.